Method and apparatus for calibrating electron-microscope magnification



Nov. 10, 1964 c. B. REIMER 3,156,820

METHOD AND APPARATUS FOR CALIBRATING ELECTRON-MICROSCOPE MAGNIFICATION Filed March 22, 1962 FIG, 1

XNVENTOR. Charles B, Palmer United States Patent M METHOD AND APPARATUS FQR CALlBRATlNG ELECTRilN-MRRQHIQPE MAGNIFHIATHUN Charles B. Reimer, Indianapolis, ind, assignor to Eli Lilly and Company, lndianapolis, End, a corporation of Indiana Filed Mar. 22, 1962, Ser. No. 181,7 6 Claims. (til. fill-49.5)

This invention relates to the electron-microscope art, and is particularly addressed to a gauge means by which the magnification of an electron microscope can be quickly and accurately adjusted to any selected one of a plurality of a magnification values.

One important object of the invention is to provide a simple apparatus by means of which the magnification of an electron microscope can be quickly set to any selected one of several values with accurate reproducibility.

Another object of the invention is to provide asimple gauge for the purpose just indicated wherein the several calibrated values of magnification available with the device are so related to one another that each value of magnification yields an image area double that of the next lower value.

A still further object of the invention is to provide, for an electron microscope, a convenient gauge for calibrating the magnification of the instrument which can be left permanently inside the instrument, usable at will in conjunction With a standard diffraction grating employed as a specimen.

Other objects and advantages of the invention will be apparent from the following detailed description of a typical embodiment thereof.

An electron microscope is a complex apparatus comprising an electron gun, a specimen holder, a complicated electron-optics system including a plurality of electron lenses, one or more cameras, and a viewing screen for visual observation of the magnified images. The depth of focus of an electron microscope is very great, and a sharp image can be obtained at almost any distance from the last lens in the optics system. The magnification, however, is of course ar'iected by this distance. Usually numerical values of magnification refer to those obtained at the position of the photographic plate carried in a cassette below the Viewing screen.

Because electrons are scattered by air molecules, the entire space inside an electron microscope is evacuated to an extremely low pressure whenever the instrument is in use.

The magnification of an electron microscope of a magnetic type is a complicated function of the electron-lens characteristics, which in turn are governed by the current magnitudes flowing through the respective electromagnet coils employed in the electron-optics system. Hence adjustment of the instruments magnification involves variation of these current magnitudes. Such adjustment is not simple, at best, and it is made more complicated by the fact that any change in magnification is accompanied by de-focusing. Readjustment of the focus, in turn, alters the magnification, so that calibration of an electron microscope to provide a sharp image at a predetermined specific magnification is a tricky and difficult job. When carried out by prior-art methods, such adjustment may well require several hours of work, even when the operator is thoroughly experienced. It should be understood that the foregoing is merely illustrative, and that the invention can be employed equally well for the calibration of electron microscopes of the electrostatic type.

When used in a busy laboratory, an electron microscope may well be operated, from one time to another, at a great variety of different magnifications. Despite this fact, it is frequently desirable to be able to adjust the instrument at will to some specific value of magnification so as to 3,156,829 Patented Nov. 10., 1964 duplicate the conditions that previously existed during some related experiment.

By the present invention, 1 have provided a simple and effective means, not only for readjusting the instrument to duplicate some previously used magnification, but also to permit quick adjustment of the instrument to any selected one of a plurality of magnifications, over a wide range of values.

A particularly useful feature of my invention is the provision of means by which all successive calibrated magnification magnitudes are related to one another by the factor /2. This arrangement is particularly useful in practical work because it means that each calibrated value of magnification provides an image area double that provided by the next higher calibrated magnification value.

Basically, the apparatus which, by my invention, is added to a conventional electron microscope consists simply of a circular electron-opaque ring, suitably mounted within the microscope interior in such manner that it may at will be thrown into the path of the electron beam or moved out of such path.

The manner in which this simple calibrating device is combined with the microscope proper, and the way it is used, me illustrated in the appended drawing, in which FIG. 1 is a simplified sectional view of the lower part of an electron microscope embodying my invention; FIG. 2 is a perspective view of the calibrating ring which forms the important part of my invention, and FIG. 3 is a fragmentary showing of the viewing screen of the FIG. 1 embodiment, illustrating the appearance of such screen when my calibrating ring is being employed in conjunction with a diffraction-grating replica as a specimen. FIG. 4 is a fragmentary view, partly in section, showing the details of a suitable adjusting mechanism for the calibrating ring.

An electron microscope is normally disposed in such a position that the electron beam is symmetrical with respect to a vertical axis, and the instrument indicated in FIG. 1 is so shown. Near the bottom of FIG. 1, viewing screen 11 may be seen, and below the viewing screen is the conventional apparatus for making photornicrographs, including the photographic plate 12 and the cassette holder 13, both of which are carried inside the photo chamber 14. Above the viewing screen 11 is a transparent panel 15, through which the image on the viewing screen 11 may be visually observed. (When photographs are to be made, the viewing screen 11 is moved out of the beam path in any conventional manner, to enable the electron beam to impinge directly on the sensitized plate 12.)

Near the top of FIG. 1, I show the lowermost electron lens in the electron-optics system, including permeable pole pieces 17, an electromagnet coil l8, and a magnetic core 19. A conduit system 29 is provided for evacuating the interior of the instrument preparatory to use.

As persons skilled in the art will readily understand, the upper part of the microscope (not shown in the drawing) includes the remainder of the electron-optics system, the specimen holder, the electron gun, and the electrical circuit components employed for electron-beam generation, acceleration, and focusing. Since all of these components of an electron microscope are conventional, they will not be illustrated or described herein.

At a suitable position in the electron drift space below the lowermost electron lens, I provide my calibrating ring 21, diagrammatically indicated in FIG. 1 and shown in perspective in FIG. 2. This ring is attached to a movable support which can be controlled by any convenient means, as a rack and pinion supported by housing 25, operable from outside the microscope, as by a sylphon. The ring projects forwardly of its supporting arm 21a, as

shown in FIG. 2, and is movable between a retracted position outside the electron beam and an operating position whereat it intercepts the electron beam, casting a shadow on the middle portion of the viewing screen ill. or the photographic plate 12, whichever is in use. The vertical orientation of the calibrating ring 21 may be fixed at any convenient height.

A suitable mechanism for moving the calibrating ring 21 between its advanced and retracted positions is shown in FIG. 4, the ring-supporting arm 21a being supported for rotation within the housing 25 and being keyed to a pinion 26. The pinion and ring-supporting arm Zita may be rotated by means of a rack arm 27, the rack portion of which is in mesh with pinion 26. At its outward end, rack arm 27 is secured to the movable face of a sylphon bellows Z3, biased by means of spring 29. An adjusting screw Ell, carried by a bracket 32, bears against the external side of the end face of bellows 28 and thus permits manually controlled movement of calibrating ring 21 between its advanced and retracted positions.

It will be understood that the apparatus of FIG. 4 is merely illustrative of one suitable means for supporting the ring 21 and moving it out of the way of the electron beam after the instrument has been calibrated. In many microscopes, the need for a separate means for moving the ring into and out of beam-intercepting position may be avoided by mounting the ring 21 on the frame of the auxiliary camera which is disposed below the lowermost lens, means being provided in such instruments for moving that camera frame out of the electron-drift zone when not in use.

As skilled readers will appreciate, the ring 21, when oriented in the electron-drift space below lens 17, will cast on the viewing screen or photographic plate a magnified shadow, the degree of magnification of such shadow depending on the vertical orientation or" the ring 21 within the drift space. Since the ring is disposed downstream of the lowermost electron lens, however, this magnification will not be large.

In a typical embodiment of the invention, the shadow cast on the photographic plate 12 by the ring 21 was magnified 4.01 diameters with respect to the ring itself. (As will be readily understood by skilled readers, the magnifi cation of the ring shadow is not afiected by the controls which govern the magnification of the specimen imag since the ring is situated downstream of the electron-optics system.)

The ring 21 may be machined from brass or other machinable, non-ferromagnetic metal; the mounting bracket 2dr: need not be integrally formed with the calibration ring proper but may be soldered or otherwise joined thereto after the ring has been machined. Fonmost effective utilization of the principles of my nvention, the rin. 21 should be so dimensioned that its 1nncr and outer diameters bear a definite mathematical relation to one another. In the preferred form of my invention, the ring 21 is so machined that its outer diameter is quite precisely equal to V3 times the inner diameter. This last-mentioned proportioning of the ring 21 permits the highly desirable feature already mentioned whereby each calibrated value of magnification selectable by means of the ring provides an image area for the specimen that is double the area afforded by the next lower calibrated value of magnification.

In using my ring 21 for calibrating the magnification of the microscope, a pre-shadowed carbon replica of a diffraction grating of known characteristics is inserted in the specimen holder, and its magnified image is projected on the viewing screen, with the ring 21 in position centrally of the electron-drift zone.

The viewing screen, as is well known to skilled readers, is coated with a phosphor which glows visibly under electron bombardment, to a degree proportional to the beam intensity. In consequence, the impingement of the electron beam on the screen 11 produces thereon a visible image of the specimen, greatly magnified.

In my invention, when the ring 21 is in its operating position, the image of the specimen is partially masked out by the shadow of the ring. This is illustrated in FIG. 3, where the specimen is a diffraction-grating replica. The magnified images of the grating lines are designated by the reference numeral 31, and the shadow of the ring 21 is designated by the reference numeral 41.

In adjusting the magnification controls of the microscope, the magnified image of the grating is kept under observation, and the magnification of the instrument is determined by noting the number of spacing intervals between grating lines that are included within either the outer diameter or the inner diameter of the ring shadow 41.

In FIG. 3, the magnification of the instrument is such that slightly less than six grating spacings are included within the outer diameter of the ring shadow 41. Adjustment of the controls to reduce the magnification slightly would reduce correspondingly the apparent spacing of the grating lines and thereby bring the magnification to a value such that six full spacings would just be included within the diameter of the ring shadow. This would represent one of the standard magnification values defined by my gauge means.

Other predetermined values of magnification may be achieved by adjusting the controls so that the ring-shadow diameter just includes other integral numbers of grating spacings, it being understood that one set of magnification values is defined by the outer diameter of the ring shadow 41 and another set, related to the first set by the factor 2 is defined by the inner diameter of the ring shadow 41.

The dimensions of the ring 21 may be determined by the following equations:

O.D.=the outside diameter of the gauge ring I.D.:the inside diameter of the gauge ring N :the integral number of grating-line spacings to be included within the outer diameter of the gauge ring at some predetermined value of magnification M :such predetermined value of magnification S the spacing between adjacent lines of the diifraction grating M the magnification of the gauge-ring shadow on the viewing screen In the embodiment of my invention already mentioned, wherein M equalled 4.01 diameters, I used a grating replica marked with 28,800 lines per inch and let N equal unity at the highest value of magnification desired, which was 32,000X.

Under these conditions, using the formulas given, I calculated that the outer diameter of the gauge ring should be 0.277 inch and the inner diameter 0.196 inch.

In that embodiment of the invention, standard values of magnifications were provided by the gauge ring as follows:

N0. of Grating Spacings Included in 1.13.

Magnification The practical value of my invention as herein disclosed has been strikingly demonstrated in the laboratory. An electron microscope modified to embody the invention can be adjusted within a few minutes to any of the standard magnifications defined by the calibrating ring, with calibration errors almost never exceeding 2% and frequently less than 1%.

It will of course be understood that, after the instrument has been adjusted to the magnification desired, the calibrating ring 21 is moved out of the way, so that its shadow is no longer superimposed on the magnified image of the specimen.

While I have in this specification described in considerable detail a typical embodiment of my invention, it is to be understood that this description is intended to be illustrative rather than limiting. The scope of my invention is to be measured primarily by reference to the appended claims.

I claim:

1. In an electron microscope having an electron-optics system from which a focused electron beam is projected, an image-receiving object such as a photographic plate, and a housing defining an electron drift space between said system and said object, the improvement which comprises an electron-opaque circular gauge element mounted in said housing and movable between an operating position in the path of said beam and a retracted position outside said beam, and means for moving said element to place said gauge element in either of said positions, the gauge element in its operative position being effective to cast a shadow on the image-receiving object suitable for use in calibrating the microscope magnification by comparison of the shadow size with the size of a magnified standard image projected on the image-receiving object by the electron beam.

2. The apparatus defined in claim 1 wherein said gauge element comprises a ring-shaped portion having an outer diameter and an inner diameter of predetermined dimensions.

3. The apparatus defined in claim 2 wherein the ratio of said outer diameter to said inner diameter is substantially equal to V2:

4. The method of calibrating the magnification of an electron microscope which comprises the steps of using as a specimen a diffraction-grating replica providing a plurality of parallel lines of uniform predetermined spacing, projecting a magnified image of such specimen, interposing in the path of such projection a circular gauge member of known dimensions, whereby the shadow of said gauge member is superimposed on the projected image of said specimen, and adjusting the magnification controls of the microscope until the diameter of said gauge-member shadow includes a predetermined integral number of spaces between said parallel lines.

5. The method of calibrating the magnification of an electron microscope which comprises the steps of using as a specimen a diffraction-grating replica providing a plurality of parallel lines of uniform predetermined spacing, projecting a magnified image of such specimen, interposing in the path of such projection a circular gauge member comprising a ring-shaped portion having an outer diameter and an inner diameter of predetermined dimensions, whereby the shadow of said ring-shaped portion is superimposed on the projected image of said specimen, and adjusting the magnification controls of the microscope until the shadow of one of said diameters embraces within its limits a predetermined integral number of spaces between said parallel lines.

6. The method defined in claim 5 wherein the ratio of said outer diameter to said inner diameter is substantially equal to V2 said method including the further step of changing the magnification of the electron microscope by adjusting said magnification controls until the integral number of spaces formerly embraced within the shadow of one of said diameters is embraced within the shadow of the other of said diameters, thereby changing by a factor of two the area of said magnified image.

References Cited by the Examiner UNITED STATES PATENTS 1,396,920 11/21 Brostrom 250--59 2,372,422 3/45 Hillier 25 049.5 2,745,966 5/56 Verhoeff 250-49.5 2,878,387 3/59 Chesterman 25049.5

RALPH G. NILSON, Primary Examiner. 

1. IN AN ELECTRON MICROSCOPE HAVING AN ELECTRON-OPTICS SYSTEM FROM WHICH A FOCUSED ELECTRON BEAM IS PROJECTED, AN IMAGE-RECEIVING OBJECT SUCH AS A PHOTOGRAPHIC PLATE, AND A HOUSING DEFINING AN ELECTRON DRIFT SPACE BETWEEN SAID SYSTEM AND SAID OBJECT, THE IMPROVEMENT WHICH COMPRISES AN ELECTRON-OPAQUE CIRCULAR GAUGE ELEMENT MOUNTED IN SAID HOUSING AND MOVABLE BETWEEN AN OPERATING POSITION IN THE PATH OF SAID BEAM AND A RETRACTED POSITION OUTSIDE SAID BEAM, AND MEANS FOR MOVING SAID ELEMENT TO PLACE SAID GAUGE ELEMENT IN EITHER OF SAID POSITIONS, THE GAUGE ELEMENT IN ITS OPERATIVE POSITION BEING EFFECTIVE TO CAST A SHADOW ON THE IMAGE-RECEIVING OBJECT SUITABLE FOR USE IN CALIBRATING THE MICROSCOPE MAGNIFICATION BY COMPARISON OF THE SHADOW SIZE WITH THE SIZE OF A MAGNIFIED STANDARD IMAGE PROJECTED ON THE IMAGE-RECEIVING OBJECT BY THE ELECTRON BEAM.
 4. THE METHOD OF CALIBRATING THE MAGNIFICATION OF AN ELECTRON MICROSCOPE WHICH COMPRISES THE STEPS OF USING AS A SPECIMEN A DIFFRACTION-GRATING REPLICA PROVIDING A PLURALITY OF PARALLEL LINES OF UNIFORM PREDETERMINED SPACING, PROJECTING A MAGNIFIED IMAGE OF SUCH SPECIMEN, INTERPOSING IN THE PATH OF SUCH PROJECTION A CIRCULAR GAUGE MEMBER OF KNOWN DIMENSIONS, WHEREBY THE SHADOW OF SAID GAUGE MEMBER IS SUPERIMPOSED ON THE PROJECTED IMAGE OF SAID SPECIMEN, AND ADJUSTING THE MAGNIFICATION CONTROLS OF THE MICROSCOPE UNTIL THE DIAMETER OF SAID GAUGE-MEMBER SHADOW INCLUDES A PREDETERMINED INTEGRAL NUMBER OF SPACES BETWEEN SAID PARALLEL LINES. 